1. Field of the Invention
The present invention relates to an object detection apparatus or an object detection method for detecting a target object in an image.
2. Description of the Prior Art
Conventionally, a human body detection method is proposed, which is for a purpose of detecting a pedestrian or an incomer, personal identification, or framing.
A human body detection method is well known in which a camera is installed in a place where pedestrians often pass by, and images taken by the camera are analyzed so that an image area of a pedestrian is detected for detecting a pedestrian. As the method for detecting the image area of a pedestrian, a method utilizing a background differential, a method utilizing a movement differential (a time differential), a method calculating an optical flow, a method detecting a flesh color area, a method detecting an oval shape of a head, and a method detecting a part of a body such as an eye or a nose are proposed, for example.
The above-mentioned detection method has a disadvantage that there is a case where a pedestrian cannot be detected properly under the following condition.
For example, if a background image has a color similar to a skin of a human body or if an error is generated in a background reference image due to a change of lighting condition, a background differential cannot be calculated correctly. In this case, the method utilizing the background differential cannot detect a pedestrian properly. If a moving speed of a pedestrian is low or if a pedestrian is motionless, difference between two images is hardly showed off. In this case, it is possible that the method utilizing a movement differential (a time differential) and the method calculating an optical flow cannot detect a pedestrian properly.
If the background area contains much flesh color, the background area can be recognized incorrectly to be a human body area. If the pedestrian faces backward, there is a possibility that the method of detecting a flesh color area cannot detect the pedestrian properly since the flesh color area is small. If the pedestrian wears a cap, glasses, sunglasses or a mask, there is a possibility that the method of detecting an oval shape of a head or the method of detecting a part of a body such as an eye or a nose cannot detect the pedestrian since the part is hidden.
As a method that can solve the above-mentioned problems, the method described in Japanese unexamined patent publication No. 11-316845 is proposed. According to this method, a plurality of subject extractions having different characteristics are performed, and the subjects extracted in each subject extraction are combined. If the result of the combination does not satisfy predetermined acceptance criteria, auxiliary subject extractions having different characteristics are further performed. Then, the result of the combination and the extraction results by the auxiliary subject extractions are used for extracting a main subject.
However, according to this method, if the main subject extracted by using the result of the combination and the extraction results by the auxiliary subject extractions still does not satisfy the predetermined acceptance criteria, further auxiliary subject extractions having different characteristics are required to perform. Therefore, depending on reliability of the result of the original subject extraction, a lot of additional auxiliary subject extractions are required to be performed. In addition, it is necessary always to decide whether or not the result of the combination satisfies the predetermined acceptance criteria. Therefore, the number of processes to be performed simultaneously (i.e., for one frame) increases, and a processing speed becomes very slow as a result.